All authentics come in numbered sizes, Such as 48, 50, 52 etc. If you get any cheaper that 200 is a good indication too. Make sure you buy from a reputable source, not from the homie out the trunk.
When I was younger, I got a players of the century Derrick Thomas, The quality was surprisingly good on that. Its still in good shape. I have seen fake authentic nfl equipment jerseys, have the numbers start falling off. So be carefull on buying on the internet.
If you are looking for a Mitchell and Ness jersey I can help you out there......I own over 40 of them and I can spot a fake real quick.......But if its for a Reebok NFL jersey you can get them from NFLShop or the 49ers team shop or your local mall......But you could also try ebay but there are alot of fakes on there......Just make sure the price is legit and the detail of the jersey....also authentics go by numbers 52.56.60 etc.....and replicas go by XL, XXL, XXXL etc.......
pretty much everyone in here has answered the question already, but I did a lot of research before copping a jersey and I ended up waiting for a sale on nflshop.com just because I wanted to make sure my 200 wasnt wasted on a fake or pre-worn jersey from China. Try www.retailmenot.com every now and then for nflshop.com coupon codes. Even legit eBay sellers cant afford to sell authentics under $100.....